Digitized data may be stored in a data management apparatus called cloud storage or file server connected to the Internet, and may be used from various apparatus such as a laptop, a tablet device and a smart phone. Although such a method of using data has high convenience, there is a risk that data may be browsed by a third party on the Internet unintentionally. Therefore, it is essential to operate information security such as encryption or access restriction on confidential data.
Patent Literature 1 discloses performing an access control on data based on attribute information.